Generally the method of forming a multi-layer paint film where an automobile body is the object being painted is carried out by forming an electrodeposited paint film on the object being painted and heating and curing the film and then forming a multi-layer paint film comprising a mid-coat paint film, a base paint film and a clear paint film. Furthermore, at the present time aqueous paints are being used for the mid-coat paints and base paints in order to cut back on volatile organic solvents (VOC).
Moreover, in recent years the method of forming a multi-layer paint film with the so-called three-coat one-bake (3C1B) system in which an aqueous mid-coat paint is coated over the electrodeposited paint film and a mid-coat paint film is formed, an aqueous base paint is coated over the uncured mid-coat paint film and a base paint film is formed, a clear paint is coated over the uncured base paint film and a clear paint film is formed and these three paint film layers are heated and cured at the same time is starting to be used from the viewpoint of energy conservation.
With the method of multi-layer paint film formation with this 3C1B system the aqueous base paint is coated over the uncured mid-coat paint film with a so-called wet-on-wet system and so there is a problem in that mixing is liable to occur between the mid-coat paint film and the base paint film and the appearance of the paint film decline.
The inclusion of an acrylic emulsion which has a specified weight average molecular weight in an aqueous mid-coat paint has been disclosed in Patent Document 1 as a means of resolving this problem.
Furthermore, the inclusion of an acrylic emulsion which has a specified glass transition temperature, acid value and hydroxyl group value and a urethane resin emulsion which has a specified acid value in an aqueous mid-coat paint has been disclosed in Patent Document 2.
Moreover, the inclusion of an acrylic emulsion in the first aqueous base paint in a method for forming a multi-layer paint film of the 3C1B type in which a first aqueous base paint, a second aqueous base paint and a clear paint are used without using an aqueous mid-coat paint has been disclosed in Patent Document 3.
However, with the methods of forming multi-layer paint films described in Patent Documents 1 to 3 there are problems in that paint films which have a satisfactorily good appearance cannot be obtained and in that sags and bubbles are liable to arise.